Blue moon

A blue moon is a second full moon in a single calendar month, which happens every two to three years. The rarity of this astronomical event derives from the length of the lunar cycle, 29.53 days,[1] so most months contain only one full moon. The usage of the term blue moon to describe it results from a misinterpretation of the traditional definition of that term in the March 1946 issue of Sky and Telescope.[2] Due to the rarity of a blue moon, the term "blue moon" is used colloquially to mean a rare event, as in the phrase "once in a blue moon".[3]
This usage is unrelated to the apparent color of the moon, which at any time of the year can be affected by atmospheric circumstances. For example, volcanic eruptions and exceptionally large fires can leave particles in the atmosphere which give the sky, and thus the moon, a tinge of blue (or other colors).
Origin
One lunation (an average lunar cycle) is 29.53 days. There are about 365.25 days in a solar year. Therefore, about 12.37 lunations (365.25 days divided by 29.53 days) occur in a solar year. In the widely used Gregorian calendar, there are 12 months (the word month is derived from moon) in a year, and normally there is one full moon each month. Each calendar year contains roughly 11 days more than the number of days in 12 lunar cycles. The extra days accumulate, so every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon. The extra moon necessarily falls in one of the four seasons, giving that season four full moons instead of the usual three, and, hence, a blue moon.
The term blue moon originates in deviations of the lunar calendar from the secular calendar. Different traditions place the extra "blue" full moon at different times:
[edit]Different definitions
In calculating the dates for Lent and Easter, the Christian clergy identified a Lenten moon. Historically, when the moons arrived too early, they called the early moon a betrayer (belewe) moon, so the Lenten moon came at its expected time.
Folklore named each of the 12 full moons in a year according to its time of year. The occasional 13th full moon that came too early for its season was called a blue moon, so that the rest of the moons that year retained their customary seasonal names.
The Maine Farmers' Almanac called the third full moon in a season that had four the blue moon.
In modern use, when 13 full moons occur in a year, usually one calendar month has 2 full moons; the second one is called a blue moon. On rare occasions in a calendar year (as happened in 2010), both January and March each have 2 full moons, so that the second one in each month is called a blue moon; in this case, the month of February, with only 28 or 29 days, has no full moon.